High temperature wear-resistant alloys are often used in the critical parts of internal combustion engines. Certain wear and corrosion resistant cobalt alloys are distributed by Deloro Stellite Company, Inc. under the trade designation Tribaloy®. Alloys within the Tribaloy® alloy family are disclosed in U.S. Pat. Nos. 3,410,732; 3,795,430; 3,839,024; and in pending U.S. application Ser. No. 10/250,205. Three specific alloys in the Tribaloy® family are distributed under the trade designations T-400, T-800, and T-400C. The nominal composition of T-400 is Cr-8.5%, Mo-28%, Si-2.6%, and balance Co. The nominal composition of T-800 is Cr-17%, Mo-28%, Si-3.25%, and balance Co. The nominal composition of T-400C is Cr-14%, Mo-26%, Si-2.6%, and balance Co.
The foregoing alloys as well as other alloys utilize a so-called “Laves” phase (named after its discoverer Fritz Laves) to increase the hardness of the alloy. In general, Laves phases are intermetallics, i.e. metal-metal phases, having an AB2 composition where the A atoms are ordered as in a diamond, hexagonal diamond, or related structure, and the B atoms form a tetrahedron around the A atoms. Laves phases are strong and brittle, due in part to the complexity of their dislocation glide processes. FIG. 1 is a photomicrograph showing irregularly shaped dendritic Laves phase particles formed by solidification of a Tribaloy® alloy.
Tribaloy® coatings and other protective coatings are sometimes applied to components that are to be used in a refractory environment associated with an internal combustion engine. For example, engine valves are often overlaid at the trim with a protective alloy for prolonging service life. Because of the regular shape of the valves, the coating can be applied with plasma transferred arc welding. With irregularly shaped components, however, plasma transferred arc welding becomes cumbersome or unfeasible. For example, sharp projections, cavities, and through holes can hinder the welding process by influencing the location at which the plasma arc is transferred to the work piece. Thermal spraying can sometimes be used to coat irregular surfaces, but it results in only a mechanically bonded coating. Mechanically bonded coatings are susceptible to spalling caused by thermal cycling. Further, thermal spraying is a line of sight process. Thus, the coating can not be applied to surfaces that cannot be reached by the spraying torch.
Many irregularly shaped parts are used in or near internal combustion engines. For instance, turbochargers can be used to improve performance of gasoline and diesel internal combustion engines. A basic turbocharger includes a turbine in the exhaust system. The turbine shares a common shaft with an air compressor in the engine's air intake system. The turbine is powered by flow of exhaust gases through the exhaust system. The turbine's power is transmitted through the common shaft to drive the air compressor, which increases the pressure at the engine's intake valves. Thus, the turbocharger improves engine performance by increasing the amount of air entering the cylinders during air intake strokes.
There are different turbocharger designs, many of which involve the use of vanes to direct the flow of exhaust gases through the turbine to improve the efficiency or other operational aspects of the turbocharger. Variable geometry turbochargers adjust their geometry to alter the way exhaust flows through the turbine in response to changing needs of the engine. For example, U.S. Pat. No. 6,672,059 discloses one example of a variable geometry turbocharger. Referring to FIG. 2 (which is a reproduction of FIG. 1 of the '059 patent), the turbine 10 comprises a turbine wheel 17 mounted on a shaft 18 inside a turbine housing 12. A volute 14 is provided to conduct exhaust gases from an internal combustion engine (not shown) into the housing 12. A plurality of vanes 22 are pivotally mounted circumferentially around the turbine wheel 17 inside the housing 12 (e.g., by pins 26 received in holes 28 on a plate 24 in the housing 12).
The vanes 22 are generally sized, shaped and positioned to direct the flow of exhaust from the volute 14 to the turbine wheel 13. Further, the vanes 22 can be pivoted to adjust flow of exhaust through the turbine 10. Each of the vanes 22 of the turbocharger illustrated in the '059 patent has an integrally formed actuation tab 30 spaced apart from the axis of the respective pin 26. Each actuation tab 30 is received in a radially angled slot 32 in a selectively rotatable unison ring 34 mounted in the housing 12 concentrically with the shaft 18. Rotation of the unison ring 34 by an actuator causes the actuation tabs 30 to pivot about the axis of the respective pin 26 so the tabs remain within their slots 32. Thus, rotation of the unison ring 34 causes the vanes 22 to pivot, thereby producing the desired change in airflow through the turbine 10.
Actuation of the vanes 22 in this manner results in stress and wear on the pins 26 and the actuation tabs 30. Reliable operation of the turbocharger requires that the vanes 22, unison ring 34, pins 26 and other turbocharger components continue to perform as designed despite being exposed to numerous high temperature cycles, the chemical environment of the engine exhaust, and the mechanical stresses associated with operation of the turbocharger.
There are many variations on the variable geometry turbocharger theme. Some examples are illustrated in U.S. Pat. Nos. 4,679,984 (pivoting vanes mounted by three pins); 4,726,744 (integrally-formed vane and vane actuator combination); 6,709,232 (vane actuated by lever arm attached to side of vane); 4,499,732 (nozzle comprising fixed vanes translated axially by pneumatic actuators to adjust flow through turbine). One common thread tying the foregoing turbocharger designs together (and numerous other turbocharger designs) is that the moveable components therein (e.g., vanes and vane actuators) are irregularly shaped (i.e., they have sharp projections, cavities and/or through holes). Further, turbochargers are illustrative of the many complex irregularly shaped components that are used throughout internal combustion engines and auxiliary systems thereof.
Although it is desirable to apply a protective high-temperature, degradation-resistant coating to these components, their irregular shapes make this difficult or uneconomical to achieve. Consequently, many irregularly shaped component parts are made by investment casting with expensive alloys. In other cases, durability may be sacrificed by using a cheaper but less resistant material to make the part.